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ION EXCHANGE APPLICATIONS IN WATER TREATMENT 12.31

gen ions and all the negatively charged ions for hydroxide ions. The hydrogen and hy-
droxides then react to form water molecules:
Cation exchange for hydrogen ions:
M + +R-H--~R-M+H +
Anion exchange for hydroxide ions:
A- + R- OH---~ R- A + OH

Hydroxides and hydrogen ions react to form water:
H + + OH- --~ H20

Separate Beds
In acid exchanges, cation resins are used in the hydrogen form to exchange hydrogen ions
for all other ions. Strongly acidic cation resins are normally used, although weakly acid
cation resins are sometimes used in conjunction with the strong acid resins to increase re-
generation efficiency in larger systems where the increased capital cost can be justified.
The cation bed is normally used as the first step in the deionization process. Otherwise,
the divalent cations would precipitate as hydroxides in the anion vessel. Some systems
used in pharmaceutical applications do practice what is called reverse deionization, usu-
ally including a softener that precedes a hydroxide form anion exchanger followed by a
hydrogen form cation exchanger.
Generally speaking, demineralization always requires cation and anion exchange. Sev-
eral varieties of each kind of resin can be selected depending on the specific water anal-
ysis, quality requirements, and operating conditions. The cation and anion resins can be
in separate vessels (separate beds or two beds) or in a single vessel (mixed beds): In the
latter, the reaction products disappear by forming water molecules, which drives the ex-
change reactions and produces a greater level of purity.
In deionization systems, anion resin is used in the hydroxide form. Most demineraliz-
ers employ strongly basic anion resins, which remove all ions, including silica and car-
bon dioxide. There are two kinds, type I and type II. The type II resins are used more fre-
quently for drinking water applications because they are functionalized with an amine that
does not impart as much of a fishy odor to the water. Type II anion resins tend to have
higher operating capacity and are more efficiently regenerated, but are less thermally sta-
ble and do not have as high a life expectancy as type I resins. Type I gelular anion resins
are most widely used in large demineralizers. The more highly porous, 4% DVB gel type
I anion resins are more resistant to organic fouling than are the 8% DVB type I resins
and are more efficiently regenerated. Their operating capacities and fouling resistance are
not as good as those of type II. Type II's better fouling resistance is due at least in part
to its failure to load organic ions in the first place.
In some systems weakly basic anion resins are used. This type of resin is only able to
neutralize acidity from mineral acid salts such as sulfates and chlorides that are created
in the cation exchange vessel. Weakly basic exchangers in general do not remove weakly
ionized substances such as carbon dioxide and silica, which is okay for some applications.
They are very efficiently regenerated and are often used in conjunction with strong base
resins in large-scale plants where the extra capital cost is more than offset by reduced
chemical costs.
Various combinations of cation and anion resins are used in the process of deminer-
alization. They can be as simple as separate vessels of cation and anion resins in a two-